Well, what diseases do you find most interesting? I guarantee you that wherever you are, if there is not an institutional expert in that disease (which seems unlikely), there is someone who is similar enough that you can take their scientific approaches and meld it into your disease of interest. Also, on your rotations, find a PI who is really good at displaying passion in their work. I mean, every PI finds their work interesting, but does that interest actually show when you talk to them about it? In my experience, if a PI has general enthusiasm for their research that is easily detectable, that enthusiasm can be quite contagious.
The learning of medical school, where you are told to do "X" or read about "Y" and take a test on "Z", is very different than research, where there is far more self-directed learning, discovery. There are also a lot more hurdles and failures associated with research than clinical medicine, but personally, I think that makes the victories than much sweeter. It is also probably hard to imagine at your stage, but the further you get along in clinical medicine, the more you realize there is little that is "high yield" and many diseases are tackled currently not by precision, but with a sledgehammer.